This invention relates to controls for electronic devices and more particularly to coded switches having less terminals than switchable positions.
With the miniaturization of electronic devices and the provision of complex circuits in extremely small packages, there are many applications in which a miniature multi-position electrical switch is required. In such applications, there may be a need for selectively connecting circuits on a printed circuit board (PCB).
For example, a switch selector is often selectively actuated by an operator of an electronic device, such as a portable two-way radio, to select a desired communication channel. Often, this switch selector is a one position-to-one terminal switch where the number of terminals correspond to the number of positions available on the switch. To select the channels of the radio, the multi-position switch is interfaced with a digital frequency synthesizer by means of a control integrated circuit (IC) which requires a binary encoding of the selected switch position. In one prior art method, a separate encoding circuit external to the multi-position switch, encodes the selected switch position by means of a variable amount of delay generated by diodes or resistors.
To save the space occupied by the multi-terminals of the multi-position switch contacting the external encoding circuit on the printed circuit board of the radio, a second prior art method mechanically incorporates the encoding circuit onto the multi-position switch to form a mechanical binary coded switch. In one mechanical binary coded decimal (BCD) 16 position switch, a five-legged or seven-legged contact rotates on top of a printed circuit board to generate sixteen distinct switch combinations corresponding to the sixteen positions via the coded outputs of four output terminals and a ground.
However, there are significant drawbacks to this mechanical BCD switch. The design goal is to design a miniaturized switch small enough to fit onto a portable radio and perform reliably. However, a low production yield of this mechanical BCD switch occurs because it is a significant mechanical challenge to align a five-legged spring contact onto five tiny printed circuit board runners with all the stack tolerances of such a small BCD switch assembly. Yet, the switch design is still not small enough. This is because the alignment and tolerance problems between the multi-legged contact and the dense printed circuit board runners of the current mechanical BCD switch cannot be made any smaller. Depending on the layout of the printed circuit board, the spring contact may even be seven legged which results in even harder alignment problems. Therefore, it is a desire to significantly improve the reliability of the present mechanical BCD switch and reduce its size and cost by eliminating the mechanical density of the conventional switch mechanism which requires a multi-legged contact and a printed circuit board with small runners.